linux-hardening - Re: [PATCH] video: fbdev: au1200fb: replace deprecated strncpy with strscpy

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Date: Wed, 20 Mar 2024 15:35:26 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Helge Deller <deller@....de>
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, 
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] video: fbdev: au1200fb: replace deprecated strncpy with strscpy

Hi,

On Wed, Mar 20, 2024 at 12:56 AM Helge Deller <deller@....de> wrote:
>
> On 3/19/24 00:46, Justin Stitt wrote:
> > strncpy() is deprecated for use on NUL-terminated destination strings
> > [1] and as such we should prefer more robust and less ambiguous string
> > interfaces.
> >
> > Let's use the new 2-argument strscpy() which guarantees NUL-termination
> > on the destination buffer while also simplifying the syntax. Note that
> > strscpy() will not NUL-pad the destination buffer like strncpy() does.
> >
> > However, the NUL-padding behavior of strncpy() is not required since
> > fbdev is already NUL-allocated from au1200fb_drv_probe() ->
> > frameuffer_alloc(), rendering any additional NUL-padding redundant.
> > |     p = kzalloc(fb_info_size + size, GFP_KERNEL);
> >
> > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Cc: linux-hardening@...r.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > Note: build-tested only.
> >
> > Found with: $ rg "strncpy\("
> > ---
> >   drivers/video/fbdev/au1200fb.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/video/fbdev/au1200fb.c b/drivers/video/fbdev/au1200fb.c
> > index 6f20efc663d7..e718fea63662 100644
> > --- a/drivers/video/fbdev/au1200fb.c
> > +++ b/drivers/video/fbdev/au1200fb.c
> > @@ -1557,7 +1557,7 @@ static int au1200fb_init_fbinfo(struct au1200fb_device *fbdev)
> >               return ret;
> >       }
> >
> > -     strncpy(fbi->fix.id, "AU1200", sizeof(fbi->fix.id));
> > +     strscpy(fbi->fix.id, "AU1200");
>
> I wonder if you really build-tested this, as this driver is for the mips architecture...
> And I don't see a strscpy() function which takes just 2 arguments.
> But I might be wrong....

I did build successfully :thumbs_up:

Commit e6584c3964f2f ("string: Allow 2-argument strscpy()") introduced
this new strscpy() form; it is present in string.h on Linus' tree.

>
> Helge

Thanks
Justin